Surface optical reflector, for space craft such as a geostationary satellite

ABSTRACT

Optical surface reflector, for a spacecraft such as a geostationary satellite. 
     The exterior surface ( 10 ) of the reflector comprises a plurality of facets ( 14 ) inclined to each other and to the interior surface ( 12 ) of the reflector. The facets ( 14 ) advantageously form a pyramid with a square base in which the angle at the apex is equal to 90°. However, other arrangements are possible without departing from the scope of the invention. The invention increases radiative capacity without significantly increasing mass and overall size.

TECHNICAL FIELD

The invention relates to an optical surface reflector adapted to bemounted on a spacecraft such as a geostationary satellite for thermalcontrol thereof.

PRIOR ART

To assure the thermal control of satellites, and in particulargeostationary satellites, it is common practice to equip them withoptical surface reflectors (OSR) made from glass coated with silver onthe side facing the satellite.

Optical surface reflectors are panels, usually plane panels, transparentto infrared radiation, with the surface facing the satellite metallized.They are intended to reflect almost all incident solar radiation and tohave as high as possible a radiative capacity, especially in theinfrared band. In other words, optical surface reflectors evacuate theheat produced by some of the onboard equipment of the satellite toguarantee thermal control thereof.

In some spacecraft, such as geostationary satellites and satellites inlow heliosynchronous orbits, achieving satisfactory thermal controlimposes the use of optical surface reflectors having a high radiativecapacity and consequently a large surface area.

In the current state of the art, the only known solution for achievingthis result consists in providing the satellites with deployablereflectors formed of a plurality of articulated panels.

However, this solution has significant drawbacks. Thus, one particularconsequence of using deployable reflectors is a significant increase inthe mass and the volume of the satellites. What is more, it results inthe presence of mechanisms whose operation in a vacuum constitutes arisk in terms of reliability.

Also, it is known in the art to increase the thermal emissivity of blackbodies by making their wall in the form of a pyramidal array consistingof the juxtaposition of a large number of small triangular surfacesinclined relative to the plane of said wall. However, in this situationthere is no constraint associated with the need to absorb none of theincident solar radiation.

Finally, the document EP-A-0 930 231 describes an interface between acomponent emitting heat and a support plate on a satellite. Theinterface comprises juxtaposed metal strips each of which originally haspyramidal or hemispherical protuberances. Fixing the component to thesupport plate crushes the protuberances. This achieves improved surfacecontact between the parts and consequently improved evacuation of heattoward the support plate by conduction.

SUMMARY OF THE INVENTION

The invention is an optical surface reflector for a spacecraft, whosenovel design increases the radiative capacity of the spacecraft withoutsignificantly changing the coefficient of absorption of sunlight,minimizing the increase in mass and volume, and avoiding the addition ofmechanisms liable to impact on the reliability of the radiator.

In accordance with the invention, this result is achieved by an opticalsurface adapted to form an exterior wall of a spacecraft, said reflectorcomprising an exterior surface adapted to face out into space and aninterior surface adapted to face the spacecraft and being characterizedin that the exterior surface comprises a plurality of facets inclined toeach other and to said interior surface.

Note that the term “facets” must be understood throughout this text asdesignating distinct surface areas of finite dimensions, regardless oftheir size. Thus the scale of the dimensions of the facets may bemillimetric, decimetric or any intermediate scale without departing fromthe scope of the invention.

Producing the exterior surface of the optical reflector in the form ofinclined facets increases the radiative surface area by 12% to 15%without significantly increasing the mass and the volume of thereflector. Moreover, this arrangement also avoids using deploymentmechanisms.

The angles between adjacent facets are chosen as a function of theangles of incidence at which solar rays are liable to impinge on thereflector, i.e. as a function of the intended orientation of saidreflector on the satellite, in order for the solar rays to be reflectedonly once by the facets of the reflectors.

Each of the facets advantageously comprises a layer of materialtransparent to infrared radiation and having an interior face adapted toface the spacecraft and covered with a metallization layer.

In this case, each of the facets preferably comprises a metallizationlayer chosen from the group comprising silver and aluminum.

Each of the facets optionally further comprises a layer of materialtransparent to the solar spectrum, such as glass.

The facets are fixed to a substrate serving as an interior support andmade from a material chosen from the group comprising glass andpolytetrafluoroethylene.

To facilitate their fabrication, the facets are preferably substantiallyplane.

The facets advantageously form projecting members with a polygonal baseand disposed in a regular array on said exterior surface.

In this case, the shape of the polygonal base of the projecting membersis chosen from the group comprising equilateral triangles, squares,rectangles, lozenges and regular hexagons.

In the preferred embodiment of the invention, the projecting members arepyramids with a square base.

In the optimum situation in which the orientation of the reflector issuch that solar rays are liable to impinge on the reflector at an angleof incidence from 45° to 90° relative to the local normal to theinterior surface of the reflector, the angle at the apex of the pyramidsis 90°.

In the case of a geostationary satellite equipped with a reflector whoseorientation is such that solar rays are liable to impinge on thereflector at an angle of incidence from 60° to 90° relative to the localnormal to the interior surface of the reflector, the angle at the apexof the pyramids is 60°.

Finally, in the situation where the orientation of the reflector is suchthat solar rays are liable to impinge on the reflector at an angle ofincidence from 30° to 90° relative to the local normal to the interiorsurface of the reflector, the angle at the apex of the pyramids is 120°.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described next by way ofillustrative and non-limiting example and with reference to the appendeddrawings, in which:

FIG. 1 is a diagrammatic view in cross section representing an opticalsurface reflector according to the invention;

FIG. 2 is a perspective view from above depicting a first embodiment ofthe invention in which the exterior surface of the reflector is formedof juxtaposed pyramids with square bases;

FIG. 3 is a view from above depicting a second embodiment of theinvention in which the exterior surface of the reflector is formed ofjuxtaposed pyramids with triangular bases; and

FIG. 4 is a view comparable to FIG. 3 and depicting a third embodimentof the invention in which the exterior surface of the reflector isformed of juxtaposed pyramids with hexagonal bases.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

An optical surface reflector according to the invention constitutes arigid panel adapted to form an exterior wall of a spacecraft such as ageostationary satellite or a satellite in low heliosynchronous orbit.

As shown very diagrammatically in FIG. 1, a panel of this kind thereforecomprises an exterior surface 10 adapted to face out into space and indirect contact with the vacuum of space and an interior surface 12adapted to face the spacecraft that is equipped with said reflector.

The functions of the optical surface reflector of the invention are,firstly, to assure maximum thermal emissivity from the interior of thesatellite toward the exterior, in particular in the infrared band ofradiation, and, secondly, to reflect most of the incident sunlight thatimpinges on the exterior surface 10.

To improve the thermal emissivity without using deployable reflectors,the invention proposes to make the exterior surface 10 in the form of aplurality of juxtaposed facets 14.

In the preferred embodiments of the invention depicted in the figures,the optical reflector has a substantially plane general configuration.In other words, the interior surface 12 of the reflector is plane andits exterior surface 10, taken as a whole, i.e. at the level of the base16 of each of the facets 14, is also plane. Note that the invention isnot limited to this arrangement and also covers the situation in whichthe optical reflector has some curvature in one or two directions. Thusin the following description the normal to the plane of the reflector orto the interior surface thereof is the local normal to that plane.

Moreover, the preferred embodiments of the invention depicted in thefigures also have the common feature that the facets 14 are all planeand oriented obliquely relative to the exterior surface 10 of thereflector taken as a whole. However, the invention is not limited tothis arrangement and also covers situations in which at least some ofthe facets 14 are curved and some facets are oriented parallel to theexterior surface 10 of the reflector taken as a whole.

As shown in more detail in FIGS. 2 to 4, the facets 14 advantageouslytake the form of isosceles triangles whose bases 16 are common to twoadjacent facets and whose apexes 18 are common to four adjacent facets14 (FIG. 2), three adjacent facets 14 (FIG. 3), or six adjacent facets14 (FIG. 4).

The facets 14 therefore form on the exterior surface 10 of the reflectorprojecting members with polygonal bases disposed in a regular array onsaid exterior surface.

In FIG. 2, the projecting members have a square base and each of them isformed by the juxtaposition of four triangular facets 14.

In FIG. 3, the projecting members have a base in the shape of anequilateral triangle and each of them is formed by the juxtaposition ofthree triangular facets 14.

Finally, in FIG. 4, the projecting members have a base of hexagonalshape and each of them is formed by the juxtaposition of six triangularfacets 14.

The scale of the dimensions of the projecting members formed by thefacets 14 may be millimetric or decimetric, it being understood that allintermediate scales are equally possible. The lower limit is determinedto avoid any risk of diffraction of sunlight.

Note that the bases 16 and the apexes 18 may also consist of parallellines, generally straight lines. In this case, the facets 14 are nolonger triangular but take the form of flat strips inclined relative tothe exterior surface 10 of the reflector, taken as a whole, to formV-section projecting members.

As shown in more detail in FIG. 1, a solar surface reflector panelaccording to the invention comprises a support structure 20 to which theindividual solar surface reflectors are stuck. The reflectors are formedon their exterior faces (facets 14) of a layer of material 22transparent to solar radiation, such as glass, and on their interiorfaces of a reflective metallization layer 24. If the dimensions allow,the metallization layer 24 and the transparent material layer 22 aredeposited by evaporation in a vacuum.

Solar radiation is therefore returned to the exterior without causingheating other than that associated with transmission losses in the glassand reflection losses at the metallic layer. Heat from the satellite istransmitted by thermal conduction from the interior surface 12 of thereflector to the support structure 20, and thence to the exteriorsurface 10, from which it may be radiated into space in accordance withLambert's laws.

The support structure 20 may be made from any material having therequired mechanical and optical characteristics. This material may inparticular be a metal, a metal alloy, or a composite material.

The transparent material layer 22 is usually glass, deposited on themetallization layer 24. The transparent material layer 22 may also bedispensed with.

The support structure 20 is generally made of glass to minimize themechanical stresses between it and the transparent material layer 22 inthe event of temperature variations.

If the transparent material layer 22 is dispensed with, the supportstructure 20 may be made from some other material, such aspolytetrafluoroethylene.

The metallization layers 24 are made, for example, of silver, aluminumor any other metal or metallic alloy having a good coefficient ofreflection in the solar spectrum.

Without departing from the scope of the invention, the metallizationlayers 24 may be deposited on the transparent material layers 22 by anymeans known to the person skilled in the art, such as deposition byevaporation in a vacuum.

In the preferred embodiment of the invention depicted diagrammaticallyin FIGS. 1 and 2, the projecting members formed by the facets 14 arepyramids with a square base whose angle at the apex, in section in amedian plane parallel to the bases 16 and passing through the apex 18,is substantially equal to 90°. This arrangement is adapted to thesituation in which the solar rays impinge on the reflector at an angleof incidence from 45° to 90° relative to the local normal to theinterior surface 12 of the reflector.

In the case of a geostationary satellite, where solar rays impinge onthe reflector at an angle of incidence from 60° to 90°, the angles atthe apex of the square-based pyramids formed by the facets 14 may bemore acute and equal to 60°.

On the contrary, if solar rays impinge on the reflector at an angle ofincidence from 30° to 90°, the angles at the apex of the square-basepyramids formed by the facets 14 are more obtuse and equal to 120°.

Compared to a prior art optical reflector having a plane exteriorsurface, a reflector as described above increases by a factor of 2.82the effective surface area of the reflector exposed to cold vacuum. Onthe other hand, thermal coupling is created between the facets 14.Accordingly, the thermal emissivity of the reflector reaches valuesclose to 1, i.e. from 0.95 to 0.99, without the volume and the mass ofsaid reflector being significantly increased.

Moreover, the surface area exposed to sunlight is not significantlyincreased relative to a prior art plane optical reflector. This surfacearea therefore remains marginal and the coefficient of absorption of thereflector is not significantly increased relative to the prior art.Moreover, in the case of sunlight impinging on the reflectors placed onthe faces at the equinox of the satellite, the paths in the glass areshorter and absorption is therefore reduced.

Without departing from the scope of the invention, instead of having asquare base, the projecting pyramids formed by the facets 14 can insteadhave a base in the shape of an equilateral triangle or in the shape of aregular hexagon, as represented as if seen from above in FIGS. 3 and 4.The shape of the bases of the projecting members may also be that of alozenge or an isosceles triangle without departing from the scope of theinvention.

The embodiments just described with reference to FIGS. 1 to 4 optimizethe radiative capacities of the radiator. Alternatively, the pyramidalprojecting members may be separated by facets 14 parallel to theinterior surface 12 of the reflector. The radiative capacities are thenlower than in the figures but remain very much higher than those ofprior art reflectors.

Finally, note that the reflector of the invention may integrate exteriorcomponents such as heat pipes. It then has a hybrid structure differentfrom that shown in the figures. An increase of the radiative surfacearea relative to the prior art from 12% to 15% is also possible in thiscase.

1. An optical surface reflector forming an exterior wall of aspacecraft, said reflector comprising: an exterior surface (10) thatfaces out into space; and an interior surface (12) that faces thespacecraft, wherein, the exterior surface (10) comprises a plurality offacets (14) inclined to each other and to said interior surface (12). 2.The optical surface reflector according to claim 1, wherein the facets(14) are at angles to each other such that substantially all incidentsolar rays are reflected only once by said facets (14) in an intendedorientation of said reflector relative to said incident solar rays. 3.The optical surface reflector according to claim 1, wherein each of thefacets (14) comprises a layer (22) of material transparent to solarradiation and having an interior face adapted to face the spacecraft andcovered with a metallization layer (24).
 4. The optical surfacereflector according to claim 1, wherein each of the facets (14)comprises a metallization layer (24) chosen from the group consisting ofsilver and aluminum.
 5. The optical surface reflector according to claim4, wherein each of the facets (14) further comprises a layer (22) ofmaterial transparent to the solar spectrum.
 6. The optical surfacereflector according to claim 1, wherein the facets are fixed to asubstrate serving as an interior support (20) and the substrate is madefrom a material chosen from the group consisting of glass andpolytetrafluoroethylene.
 7. The optical surface reflector according toclaim 1, wherein lowermost edges of each of the facets (14) aresubstantially in plane.
 8. The optical surface reflector according toclaim 1, wherein the facets (14) form projecting members with apolygonal base and are disposed in a regular array on said exteriorsurface.
 9. The optical surface reflector according to claim 8, whereinthe shape of the polygonal base of the projecting members is chosen fromthe group consisting of equilateral triangles, squares, rectangles,lozenges and regular hexagons.
 10. The optical surface reflectoraccording to claim 9, wherein the projecting members are pyramids with asquare base.
 11. The optical surface reflector according to claim 10,wherein the angle at the apex of the pyramids is 90° if solar rays areliable to impinge on the reflector at an angle of incidence from 45° to90° relative to the local normal to the interior surface (12) of thereflector.
 12. The optical surface reflector according to claim 10,wherein the angle at the apex of the pyramids is 60° if solar rays areliable to impinge on the reflector at an angle of incidence from 60° to90° relative to the local normal to the interior surface (12) of thereflector.
 13. The optical surface reflector according to claim 10,wherein the angle at the apex of the pyramids is 120° if solar rays areliable to impinge on the reflector at an angle of incidence from 30° to90° relative to the local normal to the interior surface (12) of thereflector.